How can I attribute a parameter to multiple objects?
bpy.props allows you to define your own get/set/update methods for a property.

import bpy

def update(self, context):
    #self ~ owner of the property
    for obj in context.selected_objects:
        #assigning a value to obj.property would cause a recursion!!!
        obj["custom_property"] = self.custom_property

bpy.types.Object.custom_property = bpy.props.IntProperty(update=update)


If you change the attribute custom_property for an object the change will be applied to all selected objects.

You can also use drivers to synchronize properties.